Process for recovery of soluble alkali metal hydrides from insoluble alkali metal aluminum hexahydrides

ABSTRACT

A process is provided for recovering soluble alkali metal hydride values from insoluble alkali metal aluminum hexahydrides, in the form of the soluble alkali metal aluminum tetrahydrides, by reaction with an oxygen ether. The formation of the tetrahydride is accompanied by liberation of alkali metal hydride, as a valuable product.

United States Patent Murib 5] Mar. 14, 1972 [54] PROCESS FOR RECOVERY OFSOLUBLE ALKALI METAL HYDRIDES FROM INSOLUBLE ALKALI METAL ALUMINUMHEXAHYDRIDES Inventor: Jawad H. Murib, Cincinnati, Ohio Assignee:National Distillers and Chemical Corporation, New York, NY.

Filed: Aug. 7, 1968 App]. No.: 750,742

US. Cl ..23/365, 23/204 Int. Cl ..C0lb 6/24, C0lb 6/32, COlb 6/04 Fieldof Search ..23/365, 204

[56] References Cited UNITED STATES PATENTS 3,355,262 11/1967 Beaird,Jr. et a1 ..23/365 Primary Examiner-M. Weissman Attorney-Allen A. Meyer,Jr.

[ ABSTRACT 10 Claims, No Drawings PROCESS FOR RECOVERY OF SOLUBLE ALKALIMETAL HYDRIDES FROM INSOLUBLE ALKALI METAL ALUMINUM HEXAHYDRIDES Thisinvention relates to a process for the recovery of soluble alkali metalhydride values, as alkali metal aluminum tetrahydrides, such as LiAll-land NaAlH from the insoluble alkali metal aluminum hexahydrides, such asLi AlH and Na All-l and more particularly to a process for convertinginsoluble alkali metal aluminum hexahydrides to the correspondingtetrahydrides by reaction with an oxygen ether, with liberation ofalkali metal hydride as a byproduct.

Alkali metal aluminum hexahydrides have been encountered as byproductsin certain reactions involving alkali metal aluminum tetrahydrides. Forexample, F.M. Peters, Canadian Journal of Chemistry, 42, 1755 (1964),shows the preparation of bis-trimethylamine alane All-l z2NMe from thereaction of lithium aluminum tetrahydride and trimethylamine with theformation of insoluble Li All-l as a byproduct:

3LiAlH,+2NMe 2AlH zNMe +Li AlH In this reaction, 50 percent of thesoluble hydride values (Li- AlH,) were rendered insoluble (Li AlH andhence useless as reducing agent. The hexahydride byproduct has beenshown to be completely insoluble in solvents suitable for hydridereduction, as reported by Ehrlich, J. Amer. Chem. Soc. 88 858 (1966),and by Ashby, Inorg. Chem. 5, 1615, (1966).

Accordingly, a convenient recovery of the soluble hydride values fromthe insoluble hexahydrides would be desirable. Ashby describes theconversion of the hexahydridc to the tetrahydride by reaction withaluminum and hydrogen under superatomospheric pressure and hightemperature in the presence of aluminum alkyl catalyst:

Na A1H +Al+H 3NaAll-l, This, however, is a cumbersome and costlyprocedure to carry out, especially on a large scale. Such a process isnot very attractive for commercial application because of the hightemperatures required, and because of the need as reactants for hydrogenunder high pressure and metallic aluminum.

ln accordance with the invention, a process is provided for recoveringsoluble alkali metal aluminum tetrahydrides from aluminum hexahydridesby reaction with an oxygen ether. The formation of the tetrahydride isaccompanied by liberation of alkali metal hydride, according to theequation:

In the above formulas, M is an alkali metal, and R and R are organicradicals of the specific structure described below, corresponding totetrahydrofuran or a methyl ether, and n is l or 2. The oxygen etherappears to form an adduct with the tetrahydride, containing up to 2moles of the oxygen ether to 1 mole of the tetrahydride.

This process has many advantages over the prior procedure. No metallicaluminum or gaseous hydrogen under superatmospheric pressure arerequired as a starting material. The reaction occurs under moderateconditions, even at room temperature (25 C.), without the use ofhazardous materials such as aluminum alkyls. Moreover, alkali metalhydride is produced as a valuable byproduct. The process is easilyadaptable to continuous operation.

The alkali metal tetrahydride is soluble in the organic ether, while thehexahydride and the single metal hydride byproduct are insoluble. Thisbehavior permits the isolation of the soluble tetrahydride product fromthe reaction mixture.

The reaction proceeds at room temperature. It is frequently accompaniedby an extended induction period, which may require as much as 3 hours ormore, after which the reaction proceeds at an accelerated rate. Warmingthe reaction mixture to a temperature above room temperature willshorten the induction period, and increase the overall reaction rate.Consequently, it is generally preferred in a commercial process to carryout the reaction at a temperature within the range from about 25 C. upto the boiling point of the oxygen ether at atmospheric pressure. Higherreaction temperatures can be used, in the case of low-boiling oxygenethers, by carrying out the reaction under a superatmospheric pressureappropriate to maintain the oxygen ether in the liquid phase in thecourse of the reaction. In general, it will be found convenient toreflux the reaction mixture at a suitable maximum temperature. In anycase, the reaction temperature will be below the decompositiontemperature for the desired products, the alkali metal aluminumtetrahydride and the alkali metal hydride. In view of theseconsiderations, the reaction temperature does not usually exceed aboutC.

The oxygen ethers that are employed in the process of the inventionshould be good solvents for the alkali metal aluminum tetrahydride, andnonsolvents for the alkali metal hydride. This makes it possible toseparate the two reaction products at the conclusion of the reactionsimply by filtration, or by centrifuging the alkali metal hydride, afterwhich the alkali metal aluminum tetrahydride can be separated from thesolution by distillation of the solvent at atmospheric or reducedpressure.

The oxygen ether should not contain protonic hydrogen, Le, a labilehydrogen atom. Also, its boiling point should be such that the reactioncan be carried to completion at a suitable reaction temperature at whichthe organic liquid remains in the liquid phase, either at atmospheric orat superatmospheric pressure.

The preferred ethers include tetrahydrofuran, and methyl ethers havingthe structure:

CH O(CH CH O),,CH n is a number from 0 to 4. Exemplary methyl ethersinclude dimethyl ether, dimethoxyethane, dimethyl ether of diethyleneglycol (diglyme), dimethyl ether of triethylene glycol and dimethylether of tetraethylene glycol.

The process is applicable to alkali metal aluminum hexahydrides as aclass, including for instance, sodium aluminum hexahydride, lithiumaluminum hexahydride, potassium aluminum hexahydride, cesium aluminumhexahydride, and rubidium aluminum hexahydride.

The oxygen ether can, if desired, be employed in an amount in excess ofthat required to form the 2 to 1 adduct with the alkali metal aluminumtetrahydride, so as to serve as a reaction solvent. If it is desiredthat the amount of ether present should be kept at a minimum, an inertsolvent can be added, such as for instance, aromatic hydrocarbons, suchas benzene, toluene, or the xylenes; aliphatic hydrocarbons, such ashexane, heptane or octane; thioethers, such a dimethyl sulfide anddiethyl sulfide; and cycloaliphatic hydrocarbons, such as cycloheptane,cyclohexane, and cyclopentane.

It is important that oxygen and moisture be excluded from the reactionsystem, and consequently, it is usually preferable to carry out thereaction in a closed vessel under an inert atmosphere. If the system isclosed, and the reaction is operated under reflux, an inert atmospheremay not be necessary, the reaction system being bathed in vapors of theoxygen ether and/or an inert diluent, if present.

The reaction time will be determined by the time required for theinsoluble hexahydride to be transformed into the soluble tetrahydride.This is determined by reaching a constant concentration of solublehydride. This will require from 30 minutes to as much as 2 days,depending on reaction temperature.

The insolubility of the alkali metal hydride and the solubility of thealkali metal aluminum tetrahydride in the organic ether and any inertdiluent makes it possible at the conclusion of the reaction to separatethe insoluble material from the remainder by filtration or bycentrifuging. The filtrate is then concentrated by solvent distillation,under atmospheric or reduced pressure, so as to remove the unreactedorganic ether and/or any inert diluent.

The following examples represent preferred embodiments of the invention.

EXAMPLE 1 The process of Peters (Can. J. Chem.42 1755 (1964)) wasemployed to prepare bis-trimethylamine alkane AlH -2 N(CH by reaction ofLiAll-L, and N(CH Lithium aluminum hexahydride was formed as a byproductand precipitated, as it was insoluble in the reaction mixture. The whitesolid was isolated by filtration, washed thoroughly with hexane, withdiethyl ether, and with benzene, to remove any materials solubletherein, and then stored in tetrahydrofuran at room temperature underargon. The solid material Li Ali-l density 1.l3, was heavier than thetetrahydrofuran, density 0.888, and consequently settled to the bottomof the flask. During storage after several hours time, a light solidmaterial appeared, which floated on the tetrahydrofuran, while all ofthe heavy solid material at the bottom gradually reacted, and hadentirely disappeared at the end of 24 hours, at which time reaction toform the soluble lithium aluminum tetrahydride was complete.

The reaction mixture then was filtered. The supernatant solid materialthat was removed was washed thoroughly with tetrahydrofuran, and thenpumped in high vacuum at room temperature to remove traces of thetetrahydrofuran. X-ray analysis of the light solid material establishedit as pure lithium hydride.

The filtrate was analyzed for soluble hydride, lithium, and aluminum.The observed ratio Li:Al:H was l.():1.09:3.90, in close agreement withthe theoretical value of 1:1:4 for LiAlH.,. Thus, the product waslithium aluminum tetrahydride.

EXAMPLE 2 A sample of 0.304 g. lithium aluminum hexahydride, LiQAlHs(5.64 millimoles), prepared from lithium aluminum hydride and butyllithium according to the procedure described by Ehrlich (reference citedearlier), was stirred with 13.14 g. tetrahydrofuran at room temperature.Samples of the reaction mixture were withdrawn at intervals, filtered,and the soluble hydride content determined by titration with 0.1 Nsolution of iodine in toluene. No reaction was noted until after aninduction period of 3 hours, after which reaction began, and proceededat an accelerated rate. The observed change in the conversion of LiAll-i to LiAll l. with time is shown in the table below.

TABLE I Conversion of Li AlH to LiAlH, in Tetrahydrofuran at RoomTemperature q Conversion of Reaction Time. hours Li All-l to LiAlH,

'( Based on soluble hydride content.

The reaction was stopped after 26,5 hours, inasmuch as all of thestarting solid material had disappeared in the tetrahydrofuran at theconclusion of this time, and reaction was therefore essentiallycomplete.

EXAMPLE 3 A suspension of 0.71 g. Li All-l was treated with l0 ml. ofdimethoxyethane and stirred at room temperature overnight under argon.The reaction mixture was filtered, and the clear filtrate was analyzedfor soluble lithium, aluminum and active hydride. The observedanalytical results gave the atomic ratio Li:Al:I-l of l.0:l.33:4.18,indicating that the soluble species was primarily LiAlH which is solublein dimethoxyethane. The supernatant solid material separated byfiltration was Lil-l.

EXAMPLE 4 Example 3 is repeated, substituting the dimethyl ether ofdiethylene glycol for the dimethoxyethane. The product isolated is foundto be LiAll-L. The supernatant solid material which is filtered offisLil-l.

EXAMPLE 5 Example 1 is repeated except that Na -,All-l (prepared by theprocedure of Ashly [reference citedl) is treated with tetrahydrofuran.The soluble product in tetrahydrofuran is NaAll-L. The byproduct isinsoluble sodium hydride.

Having regard to the foregoing disclosure, the following is claimed asthe invention and patentable embodiment thereof:

1. A process for the recovery of soluble alkali metal hydrid values frominsoluble alkali metal aluminum hexahydrides, consisting essentially ofreacting the alkali metal hexahydride with an oxygen ether which doesnot contain a stabile hydrogen atom and which is selected from the groupconsisting of tetrahydrofuran and methyl ethers having the structure:

CH O(CH CH O),,Cl-l in which n is a number from O to 4 at a temperatureat which reaction proceeds within the range from about 25 to about C. toform a corresponding alkali metal aluminum tetrahydride and alkali metalhydride, separating the alkali metal hydride, and recovering thecorresponding soluble tetrahydride.

2. A process in accordance with claim 1 in which the oxygen ether istetrahydrofuran.

3. A process in accordance with claim 1, in which the oxygen ether is analiphatic methyl ether having the formula CH O(CH CH O ),,CH where n isa number from 0 to 4.

4. A process in accordance with claim 3, in which the ether is adimethyl ether of a polyoxyethylene glycol having from two to eightcarbon atoms, and from two to five ether oxygen atoms.

5. A process in accordance with claim 1 in which the alkali metal islithium.

6. A process in accordance with claim 1 in which the alkali metal issodium.

7. A process in accordance with claim 1 in which the reac tion with thehexahydride is carried out in suspension in an ex cess of the oxygenether in which tetrahydride is soluble and the alkali metal hydride isinsoluble.

8. A process in accordance with claim 1 in which the reaction is carriedout in suspension in an inert diluent in which the tetrahydride issoluble and the alkali metal hydride is insoluble.

9. A process in accordance with claim I in which the reac tion iscarried out under reflux of the oxygen ether.

10. A process in accordance with claim 1 in which the tetrahydride afterseparation from the oxygen ether is washed with an inert diluent inwhich it is insoluble, and which is a solvent for the oxygen ether.

2. A process in accordance with claim 1 in which the oxygen ether istetrahydrofuran.
 3. A process in accordance with claim 1, in which theoxygen ether is an aliphatic methyl ether having the formulaCH3O(CH2CH2O)nCH3 where n is a number from 0 to
 4. 4. A process inaccordance with claim 3, in which the ether is a dimethyl ether of apolyoxyethylene glycol having from two to eight carbon atoms, and fromtwo to five ether oxygen atoms.
 5. A process in accordance with claim 1in which the alkali metal is lithium.
 6. A process in accordance withclaim 1 in which the alkali metal is sodium.
 7. A process in accordancewith claim 1 in which the reaction with the hexahydride is carried outin suspension in an excess of the oxygen ether in which tetrahydride issoluble and the alkali metal hydride is insoluble.
 8. A process inaccordance with claim 1 in which the reaction is carried out insuspension in an inert diluent in which the tetrahydride is soluble andthe alkali metal hydride is insoluble.
 9. A process in accordance withclaim 1 in which the reaction is carried out under reflux of the oxygenether.
 10. A process in accordance with claim 1 in which thetetrahydride after separation from the oxygen ether is washed with aninert diluent in which it is insoluble, and which is a solvent for theoxygen ether.